A. Field of the Invention
The field of the present invention relates generally to systems for controlling sediment in earthen basins, such as groundwater recharge, treated wastewater disposal and flood control basins. More specifically, the present invention relates to apparatuses for maintaining or improving the permeability in fluid containment basins or systems that utilize multiple sloped ridges in the basin bottom through which fluid is desired to continually percolate. Even more specifically, the present invention relates to such apparatuses that are configured to maintain or improve the permeability of such basins without requiring fluid flow to the basins to be periodically and substantially reduced or stopped.
B. Background Art
Earthen basins are commonly used to contain water for several purposes including, but not limited to, groundwater recharge of surface water, flood control and containment of municipal, industrial and agricultural waste waters. The function of these basins often rely on, or are enhanced by, the percolation of the contained water through the bottom and sides of the basin. The percolation rate of the basin is primarily controlled by the underlying soil conditions and material and by the amount and type of sediment which has settled on the surface of the basin bottom. The sediment usually becomes the controlling element, often clogging a basin so that pumping the water or fluid from the basin becomes the only economical means of draining the basin for maintenance. The subsequent removal or mixing of this clogging sediment requires the use of light and/or heavy equipment after the basin has adequately dried. Unfortunately, the equipment typically used for basin maintenance can compact the surface material, thereby requiring additional efforts to uncompact the material and return the basin to its maximum infiltration performance levels. The challenge for fluid containment basin designers and operators has been to develop a low maintenance facility without compromising percolation effectiveness.
It is well known that basin percolation is at or near the maximum rate for the first several months of operation after initial basin construction or after maintenance of an existing basin because the surface of the basin has not had time to become clogged by sediment materials. The surface clogging sediment results from several of fines, including single cell and filamentous algae, silts and clays in the irrigation/recharge water and generated by interbasin erosion (filling and levy erosion). Over time the percolation ability of the decreases as the sediment forms a virtually impenetrable clogging layer. The infiltration clogging effect of the sediment is a serious concern for all industries, business and agencies using percolation basins. Accumulated sediments limit the percolation of water through a basin and, without routine mechanical maintenance, the clogging effect will eventually render a basin's percolation ability virtually useless. As set forth in more detail in U.S. Pat. No. 6,709,199 (the full content of which is incorporated into this text as though fully set forth herein), basin owners and operators have historically used discing, ripping, scraping and combinations thereof to control and/or remove the clogging sediment layer with varying degrees of success. If the sediment was composed of inorganic material, discing or shallow mixing is often ineffective because the near surface becomes clogged with the accumulated fine-grained material. If the sediment included sufficient organic material, discing or shallow mixing without routine deep drying cycles is ineffective because the near surface becomes clogged with anaerobic microbes. Scraping and subsequent ripping can be effective, but it is costly and is typically required at least every three years.
Sediments are inorganic and/or organic particles which settle on the surface of the basin during the filling and operation of the basin. The sediments are generated and accumulated via several mechanisms including: (1) release of silt and clay from the native basin material into suspension by turbulence from the filling water in a freshly maintained or newly constructed basin; (2) wave action on the basin's perimeter side slopes; (3) settling of the suspended silt and clay contained in the influent water; and (4) settling of suspended organic materials (i.e., algae and weeds) that grow in the basin. Clays and silt-clays (fines) are deposited as a thin layer on the bottom of the basin. A layer of these fines as thin as one-eighth inch has about as much resistance to infiltration as two feet of silty sands, forty feet of sugar sands and two thousand feet of clean gravel. Over time, organics may also settle to the bottom of the basin. These settled organics also affect the infiltration ability of the basin.
The common methods of maintaining a basin and controlling the clogging effect are expensive and time consuming. All these methods first require the basin be drained and then dried. After drying, heavy equipment is normally used to access and work in the basin's bottom. The draining process sometimes requires pumping the water from the basin when the basin's bottom is significantly clogged that water will not empty by percolation. Pumping is also used when the basin's bottom is only somewhat clogged, but time is of the essence. The “Dry and Crack” Method (also referred to as the “Chip” Method) is accomplished by allowing the basin bottom to dry and crack to form “chips” with small spaces between the chips. Although the permeability of the basin is initially substantially improved, the chips soon resettle and the small spaces are soon filled with sediment and the basin becomes clogged, requiring the basin to be re-dried, sometimes as often as twice a month. The “Shallow Mix” Method requires the basin bottom be dried longer and deeper to allow mechanical equipment, such as a tractor, to drive on the bottom and use a tool, such as a disc, spring tooth, plow or other shallow mixing device, to break-up and mix the chips with the upper surface material to disperse the thin layer of clogging sediment into the upper surface material. Although this process is more effective at temporarily improving permeability, over time the mixed layer becomes increasingly impermeable and must eventually be removed with heavy equipment, such as a paddle wheel scraper. The use of heavy wheeled equipment compacts the upper portion of the basin's bottom, which is so detrimental to percolation that it is often necessary to utilize another piece of heavy equipment, for instance a tracklayer (bulldozer) with ripping shanks, to decompact or loosen the compacted upper layer. A third method, the “Deep Mixing” Method, requires the basin bottom be dried to a moisture content that allows heavy equipment, such as a tracklayer, to drive on the bottom and use a ripping shank, perhaps combined with a slip plow, or other deep mixing device. Although also effective at temporarily improving the permeability of the basin, the deeply mixed layer will likely begin to support an active anaerobic condition that, over time, will become the clogging layer and limit the percolation rate. In addition, as with the Shallow Mixing Method, the use of heavy wheeled scraping equipment compacts the upper portion of the scraped basin bottom. The cost of routine mixing and the eventual removal of large quantities of material makes the Deep Mixing Method a very expensive means of maintaining a water containment basin and creates long term constraints.
Growing concerns regarding contaminants (i.e., regulated chemicals and substances) leaching into the groundwater from percolation basins has resulted in new regulations regarding the control of erosion at construction sites where surface drainage waters flow into the basins. As is well known, eroded sediments will often adsorb or bond to common contaminants and then carry those contaminants into the containment basin. In general, the Chip, Shallow Mixing and Deep Mixing methods of basin maintenance are poor methods of contaminant control because the contaminants remain in the bottom of the basin where percolation is taking place. In fact, these three methods are somewhat in conflict with contaminant control goals because the contaminants can be easily leached, with the percolating water into the unsaturated or vadose zone, then possibly into the groundwater. When contaminant control is also required of a basin, basin maintenance becomes increasingly important and more expensive. The frequently required basin draining, drying, removal of sediments and contaminants followed by the efforts to decompact the soil require significant downtime, staff and equipment. In addition, there are concerns with air dispersal of sediments and contaminants during the basin maintenance process by the creation of dust and dust particles. The conflict of percolation effectiveness versus contaminant management usually results in basins having less effective percolation characteristics and utilizing basin maintenance methods that maintain those characteristics. Concerns regarding sediment as a basin contaminant have recently required building contractors to employ expensive on-the-jobsite sediment and other contaminant containment practices and equipment.
One such method that is used for management of contaminants is the “Minimum Scraping” Method. This method is employed when the object of the maintenance is to remove the sediment with the minimum amount of excess (i.e., disposal) material, such as when the sediment is considered to contain contaminants that could accumulate over time and become hazardous waste or result in groundwater contamination. To maintain the basin, the basin bottom is dried sufficiently to allow equipment, such as a motor grader, to drive on the bottom and windrow the thin layer of sediment into ridges. The windrowed sediments are wetted (to limit air dispersal) then scraped up by a loader into a dump truck, or similar equipment, for removal. Unfortunately, depending on soil composition and compaction from the equipment, the basin bottom can become compacted quickly, resulting in ever decreasing percolation rates between cleanings, usually resulting in the basin having to be drained by pumping rather than by percolation, which limits the use of this method due to the availability and cost of operating pumping and heavy equipment.
As set forth in U.S. Pat. No. 6,709,199, the present inventor developed a sediment control system for fluid containment basins that reduces or substantially eliminates the need to completely drain fluid from the basin and th use of heavy equipment over the permeable zones of the basin. In one embodiment of that invention, the sediment control system comprises a fluid containment basin having a plurality of basin embankments enclosing a basin bottom with a plurality of ridges and furrows on the basin bottom. Each of the ridges has at least two sides, generally formed at sloped angles, and an upper area at the top of the ridge. The furrows are located adjacent and substantially parallel to the ridges such that a furrow is disposed between and bounded by a pair of ridges. The ridges are shaped and configured, such as an inverted “V” shape, to facilitate the settlement of sediment contained in the fluid into th one or more furrows. In use, the flow of fluid into the basin is reduced on a periodic basis so that wave action washes sediment off of the upper area and sides of the ridges as the water level is lowered. Although the use of wind to generate the waves is preferred, the basin can comprise a mechanism for generating the waves. After washing of the ridges, the basin is re-filled with fluid. A substantially impermeable mat of sediment can be allowed to form in the furrows to prevent migration of contaminants contained in the fluid out of the basin. With the contaminants contained in the furrows, they can be treated or, if sufficient time is available, allowed to deteriorate into harmless or less harmful components.
Although the use of ridges and furrows in basins combined with the wave washing method of cleaning such basins has been demonstrated to work very well, some fluid containment basins are operated or otherwise constrained so as to prevent routine water level decreases and/or to decrease the effect of natural wave washing. When basins are not routinely dewatered to allow natural wind driven wave action to migrate the sediment from the ridge areas to the furrow areas, sediment clogging of the ridges will eventually occur. The operational and/or constrained conditions may include one, or a combination of, and are not limited to, the following:                (1) A basin might be relatively deep and its sides relatively steep and/or the basin relatively small so that the effect of wind driven waves on exposed ridges is diminished by virtue of the decreased velocity of the wind near the basin bottom. The sides of the relatively deep basin create a “wind shadow” that can effectively dampen the wind velocity and/or create what sailors call “dirty air”. In this condition, the bottom of the basin nearest to the incoming wind direction is likely to be in the wind shadow and receive minimal wind washing effects. The bottom of the basin furthest from the incoming wind direction is much less likely to be affected by the wind shadow and therefore will likely receive effective wind washing when the ridges are exposed during declining water levels.        (2) A basin can be configured such as an intentional recreational lake, such as for boating and/or fishing, where maintaining a high water level is desired and decreasing the water level to perform routine wave washing of the ridges is undesirable. In this condition the sediment accumulates on the ridge and furrow surfaces and eventually clogs the normally permeable ridge area.        (3) A basin may be sited in an area where adequate natural wind is unavailable part or all of the year.        (4) A “high loading” basin may receive or generate relatively large quantities of organic or inorganic sediment. A basin may receive relatively high quantities of organic sediment in situations such as a municipal or industrial wastewater treatment facility water disposal/percolation basin. In such a basin, the biological oxygen demand (BOD) may be relatively high due to entrained suspended or dissolved organic particles and/or other nutrients. The suspended particles become sediment and the dissolved organic particles and/or other nutrients become “food” for microbes and/or algae that eventually settle to the basin bottom as sediment. High levels of inorganic sediment can be generated when a basin is located in a relatively dusty area and soil or other inorganic material is blown into the basin forming clogging sediment. Another type of high loading basin is a flood control catchment basin. A flood control basin often receives runoff water containing high concentrations of street debris, including dirt (such as soil, sand, silt and clay) and organic material as is found in storm runoff water. These “high loading” conditions become problematic when the basin is not routinely or adequately dewatered and effectively wind washed.        (5) A basin may be situated where infiltration rates are relatively fast, such as 5 or 10 even 30 (vertical) feet per day. These relatively high infiltration rates normally require that the basin be dewatered and wave washed much more frequently than “normal” to prevent clogging of the ridge surface. The cost efficient operation of the basin may prevent or discourage routine wave washing of the ridges by fluctuating the water levels across the ridge surface.        (6) A basin might be constrained by relatively slow infiltration rates such as half and inch per day. This condition makes dewatering a relatively slow process.        (7) The basin volume may be relatively valuable, making it undesirable to reduce flows into the basin. Such conditions exist where basins are sited in developed areas and as infiltration demand increases, basin capacity becomes increasingly scarce and valuable. In these situations operators will often elect to spend “whatever it takes” to clean basins of clogging sediment in order to maximize basin infiltration. An example of this condition is where giant pool sweep-like machines are used to dredge the bottom of a basin located in highly developed areas. The basin bottom is cleaned while the basin is in operation. The dredging operation pumps the clogging sediment to the basin's surface and to a waste basin or somehow treats the dredged flow to separate the sediment (waste) from the water.        
What is needed are new maintenance apparatuses that are adaptable for use in “ridge and furrow” basins that are particularly configured to improve permeability of the ridges for effective percolation rates through the ridges without the need to substantially reduce the fluid inflow into and fluid level of the basin. Preferably, such maintenance apparatuses should reduce the frequency of basin maintenance, the cost of that maintenance and the need to dispose of unwanted basin materials. In addition, the maintenance apparatuses should be cost effective, minimize the amount of labor necessary for basin maintenance, reduce the amount and frequency of basin downtime and substantially prevent the air dispersal of any basin contaminants.